1. Field of the Invention
The invention herein relates to the removal of water from streams of non-corrosive gases, such as oxygen. More particularly it relates to the production of substantially water-free non-corrosive gas streams for use in the production of semiconductors, pharmaceuticals, optical fibers and similar products which cannot tolerate the presence of significant quantities of water during their manufacture.
2. Descrption of the Prior Art
There are many products which must be made in an ambient atmosphere containing non-corrosive gases, particularly oxygen. In some cases the gas stream may be entirely the specific non-corrosive gas, or in others it may be a mixture of that gas and other high purity gases or vapors, such as elemental materials and compounds which are to be deposited from the gaseous or vaporous state to form the desired product. Typically such gas/vapor streams are used in the manufacture of high purity products such as silicon wafers for semiconductors or other electronic substrates, semiconductor layer circuitry, prosthetic products for human and animal usage, pharmaceuticals, optical fibers and optical fiber devices. In order for the products to be of high purity, the gases fed to the deposition chamber must be themselves of high purity. The presence in the gas of particulates or, most importantly, water, can substantially reduce the yield of useful products from the manufacturing process.
Water is one of the most common and yet most difficult impurities to remove from the gases. Water is of course ubiquitous in almost all ambient environments. Even systems which are nominally referred to as "dry" usually have significant amounts of water, and many drying processes can reduce the moisture content of a gas only to a "minimum" which is still in the parts per million (ppm) range. However, since for many purposes water contents in the ppm range are quite acceptable, there are numerous patents and articles in the literature dealing with such types of "ppm drying processes."
In the manufacture of the above-mentioned high purity products, however, moisture contents of the depositing gases which are in the ppm range are excessively wet. To form satisfactory products, the water content of the depositing gases must be reduced to the parts per billion (ppb) range, usually down to no more than about 10 ppb. For some products, it is preferably to reduce the water content even lower, down to 5 ppb, 1 ppb or less.
(For brevity in the description in this specification, the gases which can be dehydrated by this invention will be exemplified by discussion of oxygen. It will be understood, however, that the invention is application to a number of other gases, which will be indicated below.)
There exist numerous prior art processes which can remove water down to at least the 1 ppm or 100 ppb level from oxygen gas. However, when seeking to dry oxygen streams below that level, down into the low ppb range, the typical prior art materials have been ineffective. Commonly water in the oxygen rapidly degrades the efficiency of the dehydrating agent, such that while satisfactory dehydration occurs initially, within a short time the degree of dehydration begins to decline, so the water content of the oxygen stream starts to increase and quickly reaches an unacceptable level. At that point a fresh quantity of dehydration agent must be inserted into the gas stream. This is typical, for instance, for many molecular sieve (zeolite) materials as well as compounds such as silica and alumina. The short service life of the agent makes such dehydration a costly and inefficient procedure.
Other materials which might initially be thought to be adequate adsorbers of water from gas streams based on small scale laboratory tests have been found to have too little surface area to be efficient dehydration agents on a commercial or industrial scale. Such materials become saturated very quickly and must be replaced frequently, even though the material itself is unaffected by the oxygen.
Consequently, the problem of removal of moisture down to the order of about 10-20 ppb or less from oxygen or oxygen-containing gas streams remains a significant problem in the field of large-scale commercial and industrial production of high purity semiconductors, substrates, prosthetics, ceramics, optical fibers, pharmaceuticals and the like. Those processes which are being used are expensive because of the very short service life of the dehydrating materials and the need for their frequent replacement. In addition, since it is difficult to determine the exact rate of deterioration of the dehydrating materials in the presence of the oxygen gas, user of such dehydrating materials must schedule their discard and replacement at intervals less than the shortest expected service life. To do otherwise would risk failure of a dehydrating unit with the resultant loss of contaminated product when the excessive moisture reaches the production chamber through the failed unit. Consequently, the current systems require that many if not most of the dehydrating units must be discarded while they still have some degree of useful service life left, thus further increasing the expense of the system operations.